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A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline or cenote, is a natural depression or hole in the surface topography caused by the removal of soil or bedrock, often both, by water. Sinkholes may vary in size from less than a meter to several hundred meters both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. They may be formed gradually or suddenly, and are found worldwide. These terms are often used interchangeably though many will distinguish between those features into which a surface stream flows and those which have no such input. Only the former would be described as sinks, swallow holes or swallets. A sinkhole on a glacier is termed a moulin or glacier mill.

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Mechanisms

Sinkholes near the Dead Sea, formed by dissolution of underground salt by incoming freshwater, as a result of a continuing sea level drop.

A special type of sinkhole - formed by rainwater leaking through the pavement and carrying dirt into a ruptured sewer pipe.

Mechanisms of formation may include the gradual removal of slightly soluble bedrock (such as limestone) by percolating water, the collapse of a cave roof, or a lowering of the water table. Occasionally a sinkhole may exhibit a visible opening into a cave below. In the case of exceptionally large sinkholes, such as Cedar Sink at Mammoth Cave National Park, USA, a stream or river may be visible across its bottom flowing from one side to the other.

Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by circulating ground water. As the rock dissolves, spaces and caverns develop underground. These sinkholes can be dramatic because the surface land usually stays intact until there is not enough support. Then a sudden collapse of the land surface can occur.

Sinkholes can be human-induced. New sinkholes have been correlated to land-use practices, especially from ground-water pumping, construction, and development practices. Sinkholes can also form when natural water-drainage patterns are changed and new water-diversion systems are developed. Some sinkholes form when the land surface is changed, such as when industrial and runoff-storage ponds are created. The substantial weight of the new material can trigger an underground collapse of supporting material, thus causing a sinkhole.

Occurrence

Sinkholes may capture surface drainage for running or standing water, but may also form in currently high and dry locations. The state of Florida in the USA is known for having frequent sinkholes, especially in the central part of the state. The Murge area in southern Italy also has numerous sinkholes. Sinkholes can be formed in retention ponds from large amounts of rain.

Sinkholes are usually but not always linked with karst landscapes. In such regions, there may be hundreds or even thousands of sinkholes in a small area so that the surface as seen from the air looks pock-marked, and there are no surface streams because all drainage occurs sub-surface.

Sinkholes also form from human activity, such as the rare but still occasional collapse of abandoned mines in places like West Virginia, USA. More commonly, sinkholes occur in urban areas due to water main breaks or sewer collapses when old pipes give way. They can also occur from the overpumping and extraction of groundwater and subsurface fluids.

Many sinkholes are found in Northern Michigan. These are prominent in Alpena County in Northeast Michigan. In Lachine, Michigan there are five sinkholes that are found to be very deep , and within two miles of each other. Alpena's visitor information cites their sinkholes as an attraction for visitors to the area. In August 1998 a 16 year old Alpena boy survived a 200+ ft fall in an open sinkhole 3/4 a mile off of Leer road in Lachine, Michigan (The Alpena News 8-21-1998). A majority of sinkholes in Alpena are also found underwater. Many divers explore these on a regular basis.

When sinkholes are very deep or connected to caves, they may offer challenges for experienced cavers or, when water-filled, divers. Some of the most spectacular are the Zacatón cenote in Mexico (the world's deepest water-filled sinkhole), the Boesmansgat sinkhole in South Africa, Sarisariñamatepuy in Venezuela, and in the town of Mount Gambier, South Australia. Sinkholes that form in coral reefs and islands that collapse to enormous depths are known as Blue Holes, and often become popular diving spots.

The overburden sediments that cover buried cavities in the aquifer systems are delicately balanced by groundwater fluid pressure. The water below ground is actually helping to keep the surface soil in place. Groundwater pumping for urban water supply and for irrigation can produce new sinkholes in sinkhole-prone areas. If pumping results in a lowering of groundwater levels, then underground structural failure, and thus, sinkholes, can occur.